The subject invention relates to the treatment of elastomeric copolymer films with a combination of crosslinking and surface treatments to obtain a product which will retain its transparency upon relaxation after being stretched, be free of surface tack, and display acceptable heat sealability. Such a product may be used for packaging both food and nonfood items.
The development of copolymer films useful in the packaging industry has been an active area of experimentation for many years. Currently, there are two general types of films used for packaging purposes. The first is heat-shrinkable film which, due to the temperature required to shrink-wrap such a film around a product, is undesirable for most fruit and vegetable packaging, as well as heat-sensitive nonfood packaging. The second is stretch film such as polyvinylidene chloride which lacks elastomeric properties and hence is not as desirable for packaging as are films which display more acceptable levels of elastic recovery. The object of this invention, therefore, is to provide a process whereby an elastomeric copolymer film can be treated so as to be free of surface tack, transparent upon relaxation after being stretched and heat sealable.
It is known that crosslinking elastomeric film such as ethylene/vinyl acetate copolymer (hereinafter EVA), containing about 35-50 percent copolymerized vinyl acetate by weight, by exposure to high energy ionizing radiation, results in a film with enhanced form stability, i.e., creep resistance. This crosslinking can be achieved by exposure to high energy radiation such as electrons, X-rays, gamma rays, etc., a dosage of such irradiation being measured in "rads" wherein one rad is the absorbed dose of ionizing radiation equal to an energy of 100 ergs per gram of irradiated material. A megarad (MR) is equal to one million rads. Crosslinked EVA with high vinyl acetate content (greater than 30 percent), however, is not satisfactory for stretchable film packaging due to excess surface tack which creates handling problems both in production windup and machine packaging applications.